US 2006/0219673 discloses an optical arrangement or apparatus for industrially processing a material by means of a fiber laser that emits optical radiation. The apparatus is configured to generate a ring-shaped optical power distribution on the material. In one example, the apparatus has a fiber arrangement having an optical fiber and having a multimode transport fiber with a ring shaped beam guiding profile, which are connected to one another via a coupling device. The coupling device may be configured in the form of a so-called long period fiber Bragg grating (referred to as “long period grating,” LPG) which couples the fundamental mode into at least one higher order mode of the transport fiber in order to generate the ring shaped power distribution. The apparatus described in US 2006/0219673 A1 makes it possible to generate a ring-shaped beam profile. However, the apparatus described therein does not make it possible to change the ring-shaped beam profile.
DE 10 2010 003 750 discloses a method and an arrangement for changing the beam profile characteristic of a laser beam by means of a multiply clad fiber. In the method, by means of a switchable apparatus, e.g., in the form of a movable wedge plate, which apparatus is arranged in the beam path upstream of the entrance end of the multiply clad fiber, the laser beam is optionally coupled either at least into the inner fiber core of the multiply clad fiber or at least into at least one outer ring core of the multiply clad fiber, in order to change the beam profile characteristic of the laser beam coupled out of the multiply clad fiber.
US 2009/0097807 describes an apparatus in which a single mode laser beam having an initial intensity distribution in the form of a Gaussian distribution, said laser beam emerging from a tunable laser, is converted into a laser beam having a different intensity distribution. The apparatus has a single mode fiber having a fiber core, in which the fundamental mode is guided, and also a fiber cladding surrounding the fiber core. A component arranged in line in the fiber couples a portion of the guided fundamental mode from the fiber core into the fiber cladding. The interaction between the propagation modes of the fiber core and of the fiber cladding converts the Gaussian distribution into the different intensity distribution at a predefined distance from the exit end of the fiber. The component arranged in the fiber may be a long period fiber Bragg grating. By changing the wavelength of the tunable laser, it is possible to set the proportion of the radiation which is coupled into a selected mode of the fiber cladding.
WO 2013/086227 discloses an apparatus for changing the beam parameter product of a laser beam with a waveguide. To change the focus diameter on the workpiece, it is possible to vary the input side divergence angle and/or the coupling in angle of the freely propagating laser beam into the waveguide. It is also possible to provide a divergence setting element on the waveguide in order to change the divergence of the laser beam propagating in the waveguide. The element may be, for example, a mechanical pressure generating device that generates micro-curvatures in the waveguide.
EP 1 848 074 discloses a laser apparatus having an active element having a double cladding, and also a multimode pump source to feed energy to the active element, for example to a fiber laser. To increase the coupling between the pump core and the signal core in a double clad fiber of the active element, it is proposed to add to a doped fiber length an LPG that increases the absorption of the radiation of the pump source at a predefined wavelength.
U.S. Pat. No. 8,507,877 describes a nonlinear optical system having a laser source and a light emission system having a single mode fiber, a mode converter and a multimode fiber. An LPG may serve as mode converter for exciting at least one of the higher order modes.